I am the sister rather than the parent of someone dealing with a sever form of depression, but I remember well the day my parents had to come to grips with the fact that my sister had a problem. It was something everyone in my family had to come to terms with.
I can not comprehend the fear that you must be going through, but for those around you it is actually easier sometimes to pretend that it will go away. If they can pretend, then maybe you will too and they won't have to figure out how to help you deal with something that they can't see. It is not even that they don't want to, more often is they don't know how and that is terrifying in and of its self.
If your parents are not able to comprehend what you are going through, don't let that prevent you from getting all the help you need. They may take awhile to get to the point that they understand the issues you deal with. And while it is not fair, because you need thier support, you may have to be patient with them. Do not give up on them, but right now it is more important that you get help so you can get through the day.
I know for me, watching the pain and fear and just stuff that my sister deals with has made me realize how tough you have to be somedays and I want nothing more than to help her, but we have lived with it for nine years and it took time for me to understand. I had to come to terms that her bad days are not just cause she is sad, that it is a medical condition that requires treatment, no different than someone with acute diabetes.
Meanwhile, there are those who do understand, search them out, hear their stories and know that there are many who do understand and that you are not alone in this...
Depression of any kind should not be something you are ashamed of and many people share some form of that particular burden.